Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe, and is purified therein. After, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbons (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reactions with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst has been used as such a DeNOx catalyst. The LNT catalyst adsorbs the NOx contained in the exhaust gas when air/fuel ratio is lean, and releases the adsorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich.
Since diesel engines are operated at the lean air/fuel ratio, however, it is required to artificially adjust air/fuel ratio to be the rich air/fuel ratio in order to release the adsorbed NOx from the LNT. For this purpose, a precise timing for released the NOx adsorbed in the LNT should be determined. Particularly, a NOx mass adsorbed in the LNT should be precisely determined to improve a NOx purification efficiency of the LNT and fuel economy, and to prevent degradation of the LNT.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.